The present invention relates to controlling smelt flow through and from a smelt spout of a recovery boiler to a dissolving tank.
A recovery boiler, such as a soda recovery boiler, may be used in the chemical recovery of sulfate and other sodium-based substances from pulp manufacturing processes. In the recovery boiler, waste liquor, e.g., black liquor, from the pulping process is burned to transform cooking chemicals in the waste liquor into a form suitable for the recovery process.
The waste liquor from a sulfate pulping process typically includes sodium, sulfur, organic substances and other compounds. The sodium and sulfur may be recovered using a recovery boiler. The organic substances dissolved in the waste liquor during the pulping process, e.g., digestion, are combusted in the recovery boiler. The heat produced by the recovery boiler may be used to produce steam and to melt the inorganic compounds, e.g., sodium and sulfur. The melted inorganic compounds flow as a primarily liquid smelt to the bottom of the recovery boiler.
The smelt flows from the bottom of the recovery boiler along one or more cooled smelt spouts to a dissolving tank. In the dissolving tank, the smelt is dissolved by water or weak white liquor to produce soda lye, e.g., green liquor. The main components of the smelt in a sulfate process, and the green liquor produced from it, are often sodium sulfide and sodium carbonate. The smelt produced from recovery boilers receiving waste liquor from other processes may have inorganic compounds that differ from sodium sulfide and sodium carbonate. The green liquor produced in the dissolving tank may be transported to a causticizing plant for white liquor production.
The hot smelt flow from the spout causes “banging” and explosions when the smelt falls into the cooler liquid in the dissolving tank. The banging generally results from the large temperature differential between the smelt flow and the liquid in the dissolving tank. The temperature of the smelt is on the order of 750° Celsius (° C.) to 820° C. and the temperature of the green liquor (or weak white liquor) in the dissolving tank, containing mainly water, is on the order of 70° C. to 100° C. The dramatic temperature difference between the hot smelt flow and the much cooler liquor contributes to explosions and banging nose as the smelt hits and is instantly cooled by the liquor.
The intensity of the explosive reactions of the smelt in the dissolving tank may be reduced and controlled by disrupting the smelt flow. The disruption of the smelt flow may be to breakup a smelt stream into droplets or pieces as the stream flows from the spout and before the stream hits the liquid in the dissolving tank.
It is conventional to disrupt the smelt with jet streams, e.g., conduits or steam jets, discharged from nozzles at low or medium pressure steam. These nozzles are referred to as shatter jet nozzles because they shatter the flow of the smelt. The shatter jet nozzles typically discharge a jet stream at a specific volume and rate designed to break-up the smelt flow expected during normal operation of the recovery boiler. The smelt flows at a relatively uniform rate and volumetric flow during normal recovery boiler operation.
Variations can occur in the rate and volume of smelt flowing from a recovery boiler. During normal operation of the recovery boiler, the steam jets from the shatter jet nozzles are capable of disrupting the smelt flow and sufficiently reducing explosions in the dissolving tank. However, the recovery boiler may be operated in an “upset” condition resulting in abnormal or heavy smelt flows. These heavy smelt flows may not be adequately disrupted by the jets from the shatter jet nozzle and the smelt may cause explosions from which hot smelt droplets may splatter from the tank. These excessive explosions of smelt can result in equipment damage and danger to personnel safety.